This invention relates generally to voltage to current converters.
Radio signals can include many different frequency components that are commonly referred to as channels. Usually, it is desired to select and isolate one particular channel for analog processing to be delivered to a speaker, or similar device, so that the information contained in the selected channel can be perceived by a listener as sound. In order to isolate the selected channel, radio receivers are tuned to a particular frequency, which corresponds to the selected channel.
Tuning a radio receiver requires that circuitry within the radio receiver be configured to respond primarily to a frequency corresponding to the selected channel. In earlier radio systems, tuning the radio receiver included tuning a narrow-band radio-frequency (RF) filter near the antenna input to filter the radio signal prior to that signal being amplified. The narrow filtering provided by the narrow-band filter removed essentially all frequencies from the radio signal, except for a very narrow band of frequencies around the selected channel. Passing only the frequency used for the selected channel provided for a relatively high degree of selectivity, and allowed the filtered signal to be amplified by relatively simple amplifiers.
Narrow-band RF filters, while providing good selectivity, have the disadvantage of adding expense and complexity to the radio receiver, since tuning of the narrow-band filter must be precisely coordinated with the tuning of other circuitry within the radio for optimum performance. The precise tuning requirements of narrow-band filters often require more parts with close tolerances, which can significantly add to the cost of building a radio receiver. In order to reduce the complexity and expense associated with using narrow-band filters, manufacturers have more recently begun specifying that broadband filters should be used at the antenna input in place of narrow-band filters.
The cost saving measure of using broadband filters brings with it a new set of challenges, however. Because unwanted frequencies surrounding the selected channel are not completely filtered out, greater demands are placed on subsequent portions of the radio receiver to be able to deal with extraneous frequencies and unwanted channels. For example, if a voltage-to-current converter normally used on the input to a mixer of a heterodyne receiver is not linear, additional undesired frequency components may be generated, which make it difficult for the processing circuitry to distinguish between frequency peaks associated with a desired channel, and unwanted frequency components. Prior art FIG. 1 illustrates this problem.
Prior art FIG. 1 shows a desired signal 140, a first adjacent signal 110 and a second adjacent signal 120 which are all passed through a broadband filter. It will be appreciated that generally the broadband filtering will be centered about the wanted signal 140, and that for purposes of illustration that one or more adjacent signals will generally exist on both sides of the wanted signal 140. However, for purposes of discussion, only the adjacent signals on one side of the wanted signal 140 are illustrated. In older radio systems which employ a narrow-band filter, first adjacent signal 110 and second adjacent signal 120 would be filtered out, but this is not the case when using a broadband filter. When a mixer with a voltage-to-current converter is presented with a radio signal that includes adjacent signals, such as first and second adjacent signals 110 and 120, third order signals 130 and 132 will be produced. Third order signals 130 and 132 associated with the first and second adjacent signals are unwanted artifacts produced because of non-linearities in the voltage-to-current converter that correspond to the third term in a power series equation. These third order signals, see third order signal 130, can reside at a frequency close enough to the frequency of the wanted signal 140 to cause distortion and interference. Subsequent narrow band filters in the radio circuitry will filter out first adjacent signal 110, second adjacent signal 120 and third order signal 132 without too much difficulty, because these signals have frequencies that are substantially different from wanted signal 140. However, the narrow band filters may have difficulty filtering out third order signal 130, because it is so close in frequency to wanted signal 140.
In order to make voltage-to-current converters more linear, and thereby reduce the magnitude of third order signals 130 and 132, some prior art converters have employed feedback amplifiers and diode cancellation circuits. However, these prior art attempts to make voltage-to-current converters more linear work well only over a relatively small range of frequencies, and tend not to perform well at high frequencies due to phase shift problems. In addition, extra devices and resistors can degrade noise performance. Other voltage-to-current converters have used increased amounts of bias current to obtain a greater degree of linearity. Unfortunately, in many of today""s mobile devices higher levels of bias current are impractical due to the power constraints imposed by portable power sources.
What is needed, therefore, is a voltage-to-current converter, that can be used in conjunction with broadband input filters. In particular, it would be clearly advantageous if a voltage-to-current converter could be made more linear to avoid or decrease problems with third order signals generated due to the non-linearity, while at the same time not introducing phase shift problems such as those introduced by some conventional voltage-to-current converters, degrade the overall noise figure of the receiver or use large bias currents to achieve the required linearity.